The present invention relates to a voltage reference circuit and in particular to a bandgap voltage reference circuit that eliminates the need for an error amplifier.
Bandgap voltage reference circuits are well known in the art. As is well understood, bandgap voltage reference circuits produce stable voltages that are nearly temperature independent. Stable and temperature independent voltages are useful in voltage regulators, and are commonly used in, for example, integrated circuit technology.
The fundamental principle of a bandgap voltage reference circuit is the summation of one voltage, which is proportional to absolute temperature, with another voltage, which is inversely proportional to absolute temperature. The proportional to absolute temperature first voltage is commonly generated by producing the difference between the base emitter voltages VBE of two bipolar transistor devices and is referred to as xcex94VBE. The quantity xcex94VBE can be expressed as:                               Δ          ⁢                      xe2x80x83                    ⁢                      V            BE                          =                              kT            q                    ⁢                      ln            ⁡                          (              N              )                                                          equ        .                  xe2x80x83                ⁢        1            
where k is Boltzmann""s constant, T is the absolute temperature, q is the electron charge, and N is the current density ratio of the two devices. As is well known to those skilled in the art, the current density ratio N may be generated by the two devices having a different area but the same current, or by having a different current with the same area. In bipolar technology, a device has a larger area than another device when the device has an emitter that is relatively larger than the emitter of the other device.
The voltage xcex94VBE is scaled with a multiplication factor M an d is then summed with the inversely proportional to absolute temperature second voltage. The second voltage is the base emitter voltage of a bipolar transistor and is referred to as VBE. Thus, the reference voltage can be expressed as:
VREF=M*xcex94VBE+VBExe2x80x83xe2x80x83equ. 2
The term M*xcex94VBE is often referred to as a proportional-to-absolute-temperature (PTAT) voltage VPTAT. The multiplication factor M is an analytically or empirically derived factor that adjusts the proportions of the two components of equation 2 until the temperature coefficient of the resulting sum is nominally zero.
The generation of the voltages xcex94VBE and VBE typically requires that the current in the two transistor devices is precisely the same. To ensure that the collector currents in the two devices are the same, the difference in voltages across equal collector load resistors, e.g., 56 and 58 in FIG. 2 are detected and an error term is generated. The error term is then amplified and applied to the circuit in a closed loop configuration. Thus, conventional bandgap voltage reference circuits require the use of error detection and an amplifier circuit to strictly control the operation of the circuit.
FIG. 1 shows a conventional Widlar bandgap voltage reference circuit 10, which is well known in the art. Widlar voltage reference circuit 10 includes two NPN bipolar transistor devices 12 and 14 with their bases commonly connected to the collector of transistor 12 in a current mirror configuration. The emitter of transistor 12 is connected directly to ground while the emitter of transistor 14 is connected to ground through resistor 16. The collectors of transistors 12 and 14 are connected to a current source 18 through respective resistors 20 and 22. Current source 18 is connected to a voltage source Vcc. A third NPN bipolar transistor 24, acting as an error-feedback device, has its base connected to the collector of transistor 14, while its collector is connected directly to current source 18 and its emitter connected directly to ground.
The operation of Widlar voltage reference circuit 10 is well known in the art. Widlar voltage reference circuit 10 produces a constant bandgap voltage VREF that is equal to the base emitter voltage VBE across transistor 12 and the voltage VPTAT across resistor 20. Transistor 24 shunts an amount of current from current source 18 to ground, which controls the amount of current passing through transistors 12 and 14 and thereby controls the bandgap voltage VREF. Transistor 14 is larger than transistor 12 so that the current flowing through the two transistors can be equalized, i.e., to offset the voltage drop across resistor 16. If voltage VREF begins to rise, the current passing through transistor 24 increases, which lowers VREF. If on the other hand, voltage VREF begins to decrease, the current passing through transistor 24 will decrease, which will raise VREF. Thus, transistor 24 is acting as an error amplifier, controlling the output of Widlar bandgap voltage reference circuit 10.
FIG. 2 shows another well-known bandgap voltage reference circuit. FIG. 2 is a Brokaw bandgap voltage reference circuit 50, which includes two NPN bipolar transistors 52 and 54 with their bases connected. The collectors of transistors 52 and 54 are connected to a voltage supply Vcc via respective resistors 56 and 58. The emitter of transistors 52 is directly connected to node 60, while the emitter of transistor 54 is connected to node 60 through a resistor 62. Another resistor 64 connects node 60 to ground. One input terminal of an error amplifier 66 is connected to the collector of transistor 52, while the other input terminal is connected to the collector of transistor 54. The output of error amplifier 66 produces a bandgap voltage VREF, which is fed-back to the bases of transistors 52 and 54. Thus, the output signal from error amplifier 66 provides the base current for transistors 52 and 54.
The operation of Brokaw bandgap voltage reference circuit 50 is well known to those of ordinary skill in the art. Typically in Brokaw circuit 50, resistors 56 and 58 have the same values and transistor 54 has a larger area than transistor 52 so that the currents flowing through transistors 52 and 54 are equalized and transistors 52 and 54 have an area ratio of N. During operation, error amplifier 66 attempts to equalize the current flowing through transistors 52 and 54 by forcing the voltage drop across resistors 56 and 58 to be equal. Thus, the difference in base emitter voltages xcex94VBE is equal to (kT/q)lnN as described in equation 1. The xcex94VBE term is imposed across resistor 62 that connects the two emitters of transistors 52 and 54. The resulting current, which is proportional to absolute temperature is then developed across resistor 64 thereby producing the PTAT voltage VPTAT. The bandgap voltage VREF is equal to the base emitter voltage VBE of transistor 52 plus the voltage VPTAT across resistor 64.
As discussed in reference to equation 2, the VPTAT term is equal to M*xcex94VBE. The multiplicative term M in bandgap voltage reference circuit 50 can be expressed as:                     M        =                  1          +                                    R              64                                      R              62                                                          equ        .                  xe2x80x83                ⁢        3            
where R64 is the resistance of resistor 64, and R62 is the resistance of resistor 62. Thus, the ratio of the resistances of resistors 62 and 64 can be adjusted to achieve the desired target M.
As can be seen in FIGS. 1 and 2, conventional bandgap voltage reference circuits 10 and 50 generate two voltages, a base emitter voltage VBE and the PTAT voltage VPTAT. The generation of these two voltages requires that the collector currents in the two transistors be controlled through the detection of any differences voltages across equal collector load resistors 56 and 58 in FIG. 2, producing an error term. The error term is amplified in the opposite direction known as xe2x80x9cnegative feedbackxe2x80x9d to correct the differences in the base emitter voltages.
Thus, conventional bandgap voltage reference circuits require an error amplifier, which increases the complexity of the circuit, as well as the space and cost requirements. Although the ratio of two such resistors is typically much more precise than the absolute resistance of either resistor, reducing the resulting ratio of error to acceptable levels requires the use of larger area resistors than is desirable, thus adding cost and complexity.
A bandgap voltage reference circuit includes a chain of complementary emitter follower circuits connected to a supply voltage and to common ground via respective current mirrors and connected to a proportional to absolute temperature resistor. The chain configuration of emitter follower circuits generates a summation of the base to emitter voltage differences provided by the individual emitter follower circuits within the chain. Thus, the Vptat voltage, previously generated by multiplying a single xcex94VBE by a ratio M, is replaced by a summation of multiple individual xcex94VBEs formed by each pair of complementary emitter followers such as is formed, e.g., by transistors Q1, Q2, Q3, and Q4 in FIG. 3. Consequently, by adjusting the number of emitter follower circuits along with the area of the transistors used in the emitter follower circuits, the desired PTAT voltage is generated.
The final PTAT voltage is summed with a base to emitter voltage to generate a bandgap voltage reference output signal. Because both NPN and PNP bipolar transistors are used within the bandgap voltage reference circuit, bandgap voltage reference output signals can be generated that are appropriate for both NPN and PNP based devices.
The configuration of a chain of emitter follower circuits connected to current mirrors advantageously eliminates the necessity of a gain error amplifier, which are conventionally used in bandgap voltage reference circuits. Thus, the bandgap voltage reference circuit in accordance with an embodiment of the present invention provides savings in power, cost, and complexity. The use of a number of emitter follower xcex94VBE circuits in a chain to generate the equivalent of Mxc3x97xcex94VBE eliminates the need for a precise resistance ratio. Further, the additive nature of the emitter follower xcex94VBE circuits generates lower noise and reduces process sensitivity over conventional bandgap voltage reference circuits.